Interpretive Summary: Infections by the single-celled parasite Toxoplasma gondii are widely prevalent in humans and animals. Humans become infected by ingeseting uncooked meat infected with this parasite or food contaminated with the resistant stage of the parasite (oocyst) excreted in cat feces.
Scientists at the Beltsville Agricultural Research Center and Centers for Disease Control, Atlanta, Georgia studied dynamics of T. gondii infection on a pig farm in Massachusetts and found that infections in the immediate surroundings of the pigs were the source of infection for animals on the farm. These studies will be of interest to veterinarians, parasitologists, and public health workers.

Technical Abstract:
Transmission of T. gondii infection on a pig farm in New England was investigated using genetic and ecological methods to (i) determine if infection of pigs was a result of a single source, such as in epizootic situation (e.g., outbreak) or of multiple sources, such as in an enzootic situation, (ii) identify the main source species of infection to pigs and (iii) evaluate the role of the environment surrounding the farm as the source of infection on the farm? Genetic characterization of 25 T. gondii isolates from market pigs revealed three distinct genotypes with no evidence of recombinants. These data imply that at least three distinct exposure events occurred during the seven-month lifespan of these pigs. This genotype diversity is consistent with enzootic transmission of T. gondii on the farm. Cats were suspected as the main source of pig infection based on the high seroprevalence (>95%) in pigs. The presence of the two most common T. gondii genotypes in eight isolates from free ranging chickens on this farm corroborated the role of cats because chickens were probably infected through ingestion of oocysts in the soil. The seroprevalence of toxoplasmosis in 163 wild mammals and birds captured around the pig sties (overall 13.1%) increased with proximity to the pig sties. Thus, transmission of T. gondii was higher near the pig sties than in the surrounding environment probably because increased density of oocysts there. We propose that the farm does not simply reflect its surroundings in terms of strain composition and risk of infection, but that it acts as a reservoir of strains from which the outflow of new infections into its surrounding environment is higher than the inflow.